Metal-organic frameworks (MOF) have emerged as an important class of porous materials that have the potential to be designed and functionalized for use in applications such as separations and catalysis. They have recently garnered much attention as possible materials for storing hydrogen or natural gas. Monte Carlo simulations were used to calculate nitrogen adsorption in several different classes of MOF. The BET surface areas were determined from the calculated isotherms and compared with available experimental results. The accessible surface areas directly from the crystal structures were also calculated. Comparison of the accessible surface area with the BET area from the simulated isotherms allows for a critical test of the applicability of the BET method to these ultra-high surface area materials. Layering of the adsorbed molecules within the MOF was also shown to test the underlying assumptions of the BET method. The BET and accessible surface areas were studied with respect to MOF properties such as pore size, pore shape, and free volume. These results and recommendations for the most useful definition of MOF surface areas are presented. This is an abstract of a paper presented at the 2006 AIChE National Meeting (San Francisco, CA 11/12-17/2006).